The present invention can be used anywhere for measuring the level of liquified or liquid gases, especially liquid nitrogen, liquid oxygen or liquid helium, or for detecting the presence of such liquified gases.
According to German patent application DE 39 25 927, European patent application EP 0 360 405, East German patent document DD 203 143 and U.S. Pat. No. 4,404,809, arrangements for measuring or controlling the level of liquified gases are known. Sensors, such as specially produced deposited carbon resistors, light-barriers, resistor thermometers, semiconductor diodes, swimmers and pressure gauges with heaters and contacts are used to find the level of such liquids, are known. A level control for instance consists of a three-way valve, a controlling device and a liquid gas container as well as a second liquid gas container wherein the liquid level is kept constant. A three-way valve is operated by means of a controlling device which during the filling operation keeps the liquid gas container's evaporation vent closed and by means of a heater in the container, creates pressure that feeds the liquid gas to the second container. Other arrangements do not use a heater but a pressure gas that is fed from the exterior, sometimes also by compressed air.
These arrangements have the following disadvantages:
The arrangement shown in the U.S. Pat. No. 4,404,809 needs two times four exactly selected resistors to build bridge circuits. With the arrangement used in the patent specification DD 203,143 it is possible that parts of the construction will be covered by ice which makes them nontransparent to optical beams (also infrared). Mechanical swimmers often freeze solid due to the temperatures used, and contacts often shown signs of corrosion because parts that were cooled down to low temperatures tend to steam up (water vapor in the air freezes out and thereafter defrosts). When using temperature dependent resistors as the sensors, unfortunately only small signal changes take place. The use of a traditional semiconductor diode also shows only small changes in its forward voltage depending on the temperature. The forward voltage lies at about 2 mV per 1.degree. Kelvin temperature change. The evaluation of such sensor signals is therefore very costly and strongly limits the measuring precision and reliability of the device.